For many years the very high contrast photographic images needed in the graphic arts and printing industries were obtained by developing a `lith` emulsion (usually high in silver chloride content) in a hydroquinone, low sulfite, `lith` developer by the process known as infectious development. High contrasts were achieved. However, such low sulfite developers are inherently unstable and are particularly inappropriate for machine processing.
Machine processing of graphic arts materials was achieved by the use of so called `rapid access` high contrast materials which have a toe (lower scale) contrast below 3 and typically about 2, good process latitude and good process stability. Such materials are easy to use but this is at the expense of noticeably reduced dot quality hence they are not suitable for users requiring the highest of dot qualities. These materials are, however, well accepted and widely used and are in daily use alongside nucleated products described immediately below.
Conventional rapid access films require high silver coverage to achieve the densities required e.g. for contact onto printing plates. The use of such high silver laydowns requires long development times, greater than 24s and high temperatures, greater than 34.degree. C. to ensure that all the material is fully developed and fixed.
To achieve the high image quality obtainable with lith processing and yet increase the stability of the process, emulsions containing nucleating agents, for example, hydrazides, have been used and processed in a high pH (about pH 11.5) developer with conventional amounts of sulfite, hydroquinone and possibly metol or a pyrazolidone.
A further improvement in the area of high contrast materials was the introduction of a lower pH process (below pH 11) using hydrazides active at this low pH together with the use of a contrast booster compound, for example, one of the boosters described in U.S. Pat. No. 5,316,889 or an amine booster as described in U.S. Pat. No. 4,947,354. The hydrazides proposed for use in such materials are described, for example, in U.S. Pat. No. 4,278,748, U.S. Pat. No. 4,031,127, U.S. Pat. No. 4,030,925 and U.S. Pat. No. 4,323,643 and in EP-A-0 333 435.
The use of incorporated nucleators, for example hydrazides, is not ideally desirable because the process sensitivity is still substantially worse than that obtainable in the rapid access process. This is because nucleation is a 2-phase process, an initial slow induction process followed by a rapid infectious development which will continue until all the silver is consumed or the coating is removed from the developer; hence the time of development and process activity must be controlled with great accuracy. In addition the mechanism of nucleation gives rise to chemical image spread which increases the size of exposed images and can give rise to spontaneous areas of density known as `pepper fog`.
Nucleated coatings may contain less silver than the rapid access materials but the induction period also prohibits short access times The infectious process phenomenon of `co-development` [The Journal of Photographic Science vol. 23, no. 1, 1975, LONDON(GB), pages 23-31] is defined as the tendency for unexposed silver halide grains with no latent image to develop if they are in the near vicinity of developing grains which are fogged. No spectral sensitization is described. The extent of the co-development reported has been insufficient to make this little more than an interesting observation.
EP-A-0 758 761 describes a high contrast photographic material comprising a support bearing a silver halide emulsion layer which material is free from nucleating agents and has a silver:gelatin ratio above 1 characterized in that the emulsion layer comprises both silver halide grains which are spectrally sensitized and silver halide grains which are not spectrally sensitized, and contains a density enhancing amine compound in the emulsion layer or an adjacent hydrophilic colloid layer.
When such a material is imagewise exposed, the density of the silver halide layer can be enhanced by the co-development effect to give a substantial density gain enabling the production of a high contrast material which does not contain a nucleating agent.
The examples in EP-A-0 758 761 all cite a sensitized grain to unsensitized grain ratio of 20:80 as a means to minimize dye stain whilst achieving a boost in density. However, at short development times and low developer temperatures the examples cited would not deliver high speeds and densities and would not be considered acceptable for certain graphic arts applications.
The object of the present invention is to provide improved high contrast silver halide photographic materials which do not contain a nucleating agent having improved development latitude to allow shorter development times especially at lower developer temperatures.